Conventionally a common rail system is known as a fuel injection apparatus for an internal combustion engine. The common rail system is provided with an accumulator (a common rail) that accumulates fuel therein at a specific pressure, and injects high pressure fuel supplied from the accumulator via an injector into a cylinder of the internal combustion engine. The common rail system has an excellent performance that can independently control an injection pressure and an injection amount from each other. It is demanded recently to improve the performance of the common rail system further to make exhaust gas clean and to improve a fuel consumption performance. U.S. Pat. No. 5,622,152 and its counterpart JP-2885076-B2 disclose a fuel injection apparatus to satisfy this demand in a simple fashion
The fuel injection apparatus disclosed in U.S. Pat. No. 5,622,152 is provided with; a hydraulic control mechanism for opening and closing the nozzle, which is an advantage of the common rail system; and a pressure increasing mechanism to increase a fuel pressure in the accumulator. The pressure increasing mechanism enables fuel injection at still higher pressure, and both of pressure increasing control and fuel injection control. As a result, the fuel injection apparatus can change fuel injection pressure during one injection cycle, to realize a micro injection at a low pressure and a main injection at a super high pressure, and to optimize a pattern of an injection ratio. Accordingly, further minute optimization of fuel combustion is achieved.
In the above-mentioned fuel injection apparatus disclosed in U.S. Pat. No. 5,622,152, however, it is substantially necessary to independently control two operations, that is, the pressure increasing operation and the fuel injection operation from each other. Thus, the fuel injection apparatus requires at least two actuators, for example, to make a construction of the system intricate, and to increase a manufacturing cost thereof.
In this regard, JP-2003-106235-A2 discloses another fuel injection apparatus that can achieve functions equivalent to those of the above-mentioned fuel injection system (disclosed in U.S. Pat. No. 5,622,152 and JP-2885076-B2).
FIG. 11 depicts a hydraulic circuit of the fuel injection apparatus disclosed in JP-2003-106235-A2. The fuel injection apparatus has a control valve 100 that is driven by one actuator. The control valve 100 is connected via a fuel passage 130 to a booster 110, via a fuel passage 140 to a nozzle 120, and via a fuel passage 150 to an accumulator 160. The control valve 100 is provided with: a hydraulic pressure port 101 that is connected to the fuel passages 130, 140; and a low pressure port 102 that is connected to a low pressure side drain passage 170. A valve body 103 is driven between: a valve closing position (the position shown in FIG. 11) to block a communication between the hydraulic pressure port 101 and the low pressure port 102; and a valve opening position to allow the communication between the hydraulic pressure port 101 and the low pressure port 102.
When the valve body 103 is driven to the valve closing position, fuel pressure in the accumulator 160 is transmitted to a control chamber 111 of the booster 110 and a back pressure chamber 121 of the nozzle 120. In the booster 110 in this time, the hydraulic pressure is in balance between an upstream and downstream sides of a hydraulic piston 112, which is installed in the booster 110. Thus, the pressure of the fuel, which is supplied from the accumulator 160 via a fuel passage 180 to the pressure increase chamber 113, does not increase. Concurrently, in the nozzle 120, a needle (not shown), which is installed therein and receives the fuel pressure in the back pressure chamber 121, keeps a valve closing state, not to perform fuel injection.
When the valve body 103 is driven to the valve opening position, the hydraulic pressure port 101 and the low pressure port 102 of the control valve 100 is communicated with each other, so that the fuel pressure in the control chamber 111 and in the back pressure chamber 121 is released via the control valve 100 to a lower pressure side. Thus, in the booster 110, the hydraulic pressure comes out of balance between the upstream and downstream sides of the hydraulic piston 112, to move the hydraulic piston 112 downward in the figure, so that the pressure of the fuel in the pressure increase chamber 113 increases, and the fuel is supplied to the nozzle 120. In the nozzle 120, a fuel pressure decrease in the back pressure chamber 121 lifts the needle upward, to inject the super high pressure fuel supplied from the booster 110.
In the above-mentioned fuel injection apparatus disclosed in JP-2003-106235-A2, the control chamber 111 of the booster 110 and the back pressure chamber 121 of the nozzle are connected to the accumulator 160 at all times. That is, the control chamber 111 and the back pressure chamber 121 are respectively in communication with the accumulator 160 at all times regardless of a valve opening and closing state of the control valve 100. Accordingly, the fuel passages 130, 140 and 150 are respectively provided with apertures 190, 200 and 210. However, it is difficult to optimize controls of the booster 110 and the nozzle 120 because of an interaction among the apertures 190, 200 and 210.